Environmental protection is of increasing concern in the United States and other developed nations. Control of airborne pollutant accumulation is an instituted goal of strict government regulations.
Methods of pollutant removal from air include adsorption on particulate solids such as activated carbon, absorption in liquid media, filtration, electrostatic precipitation, incineration, chemical reaction, condensation, and the like.
Aqueous solutions are utilized to absorb toxic gases such as HCl, HF, NH.sub.3, Cl.sub.2 and H.sub.2 S. Chemical means of air decontamination include oxidation of toxic chemicals with oxidizing agents such as chlorine, chlorine dioxide, hypochlorite, ozone, peroxide, and the like.
Both physical and chemical methods of air decontamination have various drawbacks. Many of the methods involve the use of chemical reagents which per se are hazardous to humans. A major concern with decontamination methods is the creation of a toxic waste disposal problem.
Of particular interest with respect to the present invention are industrial operations in which highly toxic gases such as phosphine, arsine or hydrogen sulfide are released into the environment.
Phosphine has become a fumigant of choice for foodstuffs, tobacco and related materials because of low cost and ready availability, and its comprehensive pesticidal activity. Phosphine gas typically is generated at point of use by exposure of magnesium phosphide or aluminum phosphide to ambient humidity: EQU Mg.sub.3 P.sub.2 +6H.sub.2 O.fwdarw.2PH.sub.3 +3Mg(OH).sub.2 EQU AlP+2H.sub.2 O.fwdarw.PH.sub.3 +AlO(OH)
Release of phosphine gas into the atmosphere must be minimized or prevented because of its toxicity. The tobacco industry mainly has employed silver-impregnated carbon as an adsorbent for phosphine. This type of adsorbent is limited by the amount of phosphoric acid oxidation byproduct which it can accumulate before deactivation occurs, and the silver catalyst needs to be regenerated or replaced.
Phosphine also is employed in the semiconductor industry. JP 63-87519 discloses flame combustion as a means of phosphine removal from air. U.S. Pat. No. 5,024,823 discloses the use of copper sulfate-impregnated silica gel or alumina to absorb phosphine.
U.S. Pat. No. 5,207,877 discloses a liquid scrubber which uses a redox pair of oxidant metals to oxidize and destroy volatile organic compounds or airborne microorganisms. The oxidant metals are reoxidized by electrochemical means.
The electronics industry also utilizes arsine gas in the manufacture of semiconductors. Scrubber solutions which use permanganate or hypochlorite reagents to remove arsine or phosphine contaminants from environmental air are not economical, and pose a toxic waste disposal problem.
There is a continuing need for new and improved methods and systems for removal of toxic contaminants from environmental air.
Accordingly, it is an object of this invention to provide a liquid scrubber system which is adapted for efficient removal of contaminants from air or gas by oxidation of the contaminants to water-soluble derivatives, and/or to water-dispersible derivatives which remain associated with the aqueous phase.
It is another object of this invention to provide a process for oxidative removal of phosphine and other toxic contaminants from a gas-phase by the use of a scrubber solution which includes a content of redox-effective quantities of silver(I) catalyst, hydrogen peroxide, and a metal co-catalyst which promotes reoxidation of silver metal.
Other objects and advantages of the present invention shall become apparent from the accompanying description and Examples.